The answers are
2 (2!: 2 choices for the first letter, one for the second letter)
6 (3!: 3 choices for the first letter, 2 for the second letter, 1 for the last letter)
24 (4!: 4 choices for the first letter, 3 for the second letter, 2 for the third letter, 1 for the last letter)
120 (5!: I'm too lazy to type this out)